1. Field of the Invention
The present invention relates to a recording method of high density optical recording, and more particularly, to an adaptive recording control method of independently controlling the power levels of a first pulse, a multiple pulse, and a last pulse used with optimum recording.
2. Description of the Related Art
Phase-change optical disks widely used as high density optical disks record digital signals using a recording layer in a liquid crystalline (melting) state which has the property of being able to exist in two different phases depending on the cooling rate: a crystalline phase and an amorphous phase. The temperature of the recording layer is adjusted by the power of a laser beam radiated thereon, and the cooling rate is controlled to be fast enough to result in an amorphous state, or slow enough to result in a crystalline state. That is, the phase-change optical disks record and erase digital information by adjusting the power of a laser beam incident on the recording layer and controlling the cooling rate.
In digital versatile disc-random access memories (DVD-RAMs) adopting eight to fourteen modulation plus (EFM+), marks on the recording medium representing recorded data have lengths of 3T to 11T or 14T where 1T denotes a clock period of the recording mark. Phase-change optical disks record data by mark edge recording or mark position recording. While mark edge recording allows for high density recording compared with mark position recording, it may degrade the quality of a recorded signal since a trailing edge, which is the end of a recording mark, tends to be larger than a leading edge, which is the beginning of the recording mark, thereby forming a teardrop shape.
In efforts to overcome the above problems, a method of forming a recording mark using a multiple pulse train has been proposed. The multiple pulse train comprises a first pulse, a multi-pulse, and a last pulse, wherein each pulse could have one of three levels, i.e., a peak power Pw, an erase power Pe, and a bias power Pb.
FIG. 1A is a waveform of a conventional multiple pulse train corresponding to a recording mark of a length of 3T. FIG. 1B is a waveform of a conventional multiple pulse train corresponding to a 14T recording mark. The waveforms shown in FIGS. 1A and 1B conform to 2.6 GB DVD-RAM standards.
As shown in FIG. 1A, the multiple pulse train forming a recording mark of 3T is divided into three pulse areas: a preheating pulse Tp, a first pulse Tf and a last pulse Tl. As shown in FIG. 1B, the multiple pulse train forming a recording mark of 14T is divided into five pulse areas: a preheating pulse Tp, a first pulse Tf, a multi-pulse Tm, a last pulse Tl, and a cooling pulse Tcl. The preheating pulse Tp has an erase power level of Pe and is selectively set to erase previously written content and preheating a recording layer. The first pulse Tf has a peak power level of Pw to form the leading edge of a recording mark. The multi-pulse Tm is applied between the first and last pulses Tf and Tl in forming a recording mark of 4T or greater, and cyclically alternates between the bias power level Pb and the peak power Pw. The number of pulses constituting a multi-pulse Tm corresponds to the length of the recording mark. The multi-pulse reduces non-uniformity of a long recording mark caused by thermal buildup. The last pulse Tl has peak power level Pw forming the trailing edge of a recording mark. The cooling pulse Tcl has a bias power level Pb, at which the laser power is switched off to prevent the recording mark from being too long. Unlike a 2.6 GB DVD-RAM, the power level of cooling pulse Tcl can be made different in a 4.7 GB DVD-RAM. That is, the temperature of a recording layer may be set to between 100-200° C. instead of room temperature during a cooling pulse Tcl period.
FIG. 2 shows the write characteristics of a phase-change optical disk recording medium. In the phase change disk, recording of digital information is accomplished by melting a recording layer to be in a liquid crystalline state by applying heat and then cooling it to be in a crystalline or amorphous state. A laser diode is used to apply heat. The recording layer is typically in an erase state at about 300° C. and in a liquid crystalline state above 600° C.
As is evident from a graph at the upper part of FIG. 2, if the temperature of a recording layer is at about 300° C., the recording layer is in an erase state, in which previously recorded information is erased. If the temperature rises above 600° C., the recording layer is completely in a liquid crystalline (melting) state. Thereafter, desired digital information can be recorded by adjusting the cooling rate. Cooling is accomplished by natural cooling through a substrate supporting the recording layer after the power of the laser diode is lowered (in 4.7 GB DVD-RAM) or switched off (in 2.6 GB DVD-RAM).
The lower part of FIG. 2 shows the recording state of the recording medium with respect to changes in temperature as shown in the graph of FIG. 2. Information previously written on the recording layer is erased when the recording layer is in an erase state, whereas a domain corresponding to a recording mark is formed when it is in a liquid crystalline and cooling state. Here, the domain refers to a portion corresponding to a mark when viewed from above.
The shape of the domain significantly affects the quality of signals, in particular, jitter, cross erase and cross-talk. In particular, forming the beginning, the middle, and the end portions of a domain is significantly affected by changes in temperature of a recording layer. Unless the leading and trailing edges of a domain are smoothly formed, jitter increases. If the shape of a domain widens near the middle like the domain shown with a dotted line, cross erase occurs between adjacent tracks during recording, while cross-talk occurs between adjacent tracks during reproducing.
To prevent the domain from widening in the middle, intermittent pulses are continuously applied between the leading and trailing edges of the domain (multi-pulse). The multi-pulse prevents a mark from widening in the middle by reducing thermal buildup at the middle of the mark.
However there remains room for improving recording control so as to further increase signal quality as recording density increases.